Method for improving the output power of a non-linear power amplifier

ABSTRACT

A method for improving the output power of a non-linear amplifier, wherein signal amplitudes of a digital baseband are predistorted and then fed at the input of the power amplifier, having a non-linear characteristic curve, in order to linearize the characteristic curve. The signal amplitudes are fed to the input of the power amplifier in a predistorted manner up to a predetermined transition power while being fed to the input of the power amplifier in a non-predistorted manner from the predetermined transition power on.

FIELD OF TECHNOLOGY

The present disclosure relates to a method for improving the outputpower of a non-linear power amplifier (PA), with signal amplitudes of adigital baseband being predistorted and then injected at the input ofthe power amplifier, which has a non-linear characteristic amplifiercurve, in order to linearize the characteristic amplifier curve (i.e.,predistortion).

BACKGROUND

In many areas, such as in multimode mobile telephones, a linear signalamplification with high power efficiency is needed. One option forachieving a linear signal amplification is to linearize a non-linearpower amplifier which thus exhibits a non-linear characteristic curve,by injecting a predistorted signal. This amplification process is knownas predistortion. The combination of the predistorted injected signaland the non-linear characteristic amplifier curve produces overall alinear characteristic curve of the power amplifier and thereby a linearsignal amplification.

A problem with this method of amplification is that only the signalquality is improved with particular regard to linearity. The outputpower and the efficiency have not been able to be increased thus farwith this linear signal amplification by the predistortion.

SUMMARY

Accordingly, the present disclosure provides a method under an exemplaryembodiment that allows an increase in the output power and also anincrease in the efficiency of a non-linear power amplifier andsimultaneously achieves a good signal quality.

Amplification procedures that use predistortion, which have previouslyhaving been used for complete linearization of a non-linearcharacteristic amplifier curve, can also be used by modifying them forvariable formation of the characteristic curve of the power amplifier.

Accordingly, an exemplary method is disclosed for improving the outputpower of a non-linear power amplifier (PA), in which signal amplitudesof a digital baseband are predistorted and then injected at the input ofthe power amplifier which has a non-linear characteristic amplifiercurve in order to linearize the amplifier curve (predistortion). Thesignal amplitudes are injected predistorted to a predeterminedtransition power into the input of the power amplifier, and from thispredetermined transition power, the signal amplitudes are injectednon-predistorted into the input of the power amplifier.

This configuration makes it possible for the non-linear characteristicamplifier curve to be only partly linearized. As such, this partlinearization increases the power and the efficiency of the amplifiercompared to the power at full linearization. The part linearizationminimally reduces the signal quality, but in return it makes it possiblefor the procedure to set the range of the linearity so that the signalquality is sufficiently good.

It is also beneficial for the transition power, at which the amplifiercharacteristic curve moves from a linear area into a non-linear area, tobe set. The linear area is designated as the area of the amplifiercharacteristic curve in which the output power is directly proportionalto the input power.

By setting the transition power, the linearity of the signal can beincreased or reduced. This makes it possible to vary the signal qualitywith respect to the linearity. This allows an optimum amplifier range tobe set in which the minimum required signal quality and at the same timethe maximum output power of the amplifier are achieved.

If, for example, the transitional power is reduced, this also reducesthe linear area of the characteristic amplifier curve and thereby thesignal quality, with the output power of the amplifier characteristiccurve being increased in this area. If, on the other hand, thetransitional power is increased, the linear area of the amplifiercharacteristic curve is also expanded and the signal quality therebyimproved, with the output power of the amplifier characteristic curvesimultaneously being reduced in this area. If the transitional power isset to the value of the maximum input power of the amplifier, alinearized characteristic curve is obtained.

Under a preferred embodiment, look-up table is used to applypredistortion to the signal amplitudes up to the predeterminedtransitional power and to inject the signal amplitudes as from thispredetermined transitional power into the input of the amplifier.Predistortion factors are stored In the look-up table which predistortthe small to medium signal amplitudes and inject the large signalamplitudes undistorted into the input of the amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

The various objects, advantages and novel features of the presentdisclosure will be more readily apprehended from the following DetailedDescription when read in conjunction with the enclosed drawings, inwhich:

FIG. 1 illustrates a circuit diagram of a Complex Gain DigitalPredistortion amplifier that utilizes predistortion with a complexmultiplier;

FIG. 2 is a characteristic transformer curve for differentlinearizations of the amplifier of FIG. 1, in which the output power isplotted against input power;

FIG. 3 is a graphically plotted look-up table with values of thepredistortion factors;

FIG. 4 is a diagram in which the output power is plotted against thefrequency with complete predistortion and with no predistortion of thesignal, and a table which contains the measured values for the outputpower ACLR (ACLR low and ACRL up) and the efficiency n for the case ofcomplete predistortion and no predistortion; and

FIG. 5 is a diagram in which the output power is plotted against thefrequency with partial predistortion and with no predistortion and atable which contains measured values for the output power ACLR and theefficiency for the case of complete and partial predistortion.

DETAILED DESCRIPTION

FIG. 1 illustrates a basic diagram of how, with the aid of the method inaccordance with the invention, the power and the efficiency of a poweramplifier PA 1 can be increased.

With the aid of a look-up table 5, the complex I/Q signals 4 of thebaseband, indicated by the arrow x, are multiplied by a complexmultiplier 6 with different predistortion factors. Viewedmathematically, after the complex multiplier 6 the complex envelope ofthe signal 2, indicated by the arrow y is obtained as follows:y=LUT(|y|)·x

The contribution of the complex I/Q Signal of the baseband 4.B,indicated by the arrow |x|, is thus multiplied by the variouspredistortion factors of the look-up table 5 and then with the complexI/Q signal of the baseband 4, indicated by the arrow x. The complexenvelopes of the signal 2, indicated by the arrow y, are injected at theinput of the power amplifier PA 1. For example a W-CDMA (Wideband CodeDivision Multiple Access) signal can be used as a signal. Thepredistortion allows the output signal of the line amplifier 3 availableat the output of the power amplifier PA 1, indicated by the arrow z, tobe varied as regards its linearity curve. In which case the complexenvelope of the signal 3 arrow z is produced at the output of theamplifier PA 1 as followsz =Gain _(PA)(|Y|)o y

This can be implemented overall by the combination of predistortedsignals and non-linear amplifier characteristic curves. This combinationproduces a linear signal curve. Different amplifier characteristiccurves are shown in FIG. 2.

FIG. 2 is an illustration in which different characteristic amplifiercurves are depicted with different linearization. On the abscissa of thediagram the input power Pin in [dBm] is plotted, with the abbreviationin square brackets standing for decibels in relation to one Milliwatt.On the ordinate the output power Pout is plotted, also in [dBm]. Threecharacteristic curves with three different signal linearizations areshown in the exemplary embodiment.

The topmost curve of FIG. 2 shows a characteristic amplifier curve 7.OLwithout linearization. In the area of Pin: −15 [dBm] to circa 1 to 2[dBm] this curve exhibits a linear behavior, meaning that the inputpower Pin is proportional to the output power Pout. In the area of Pin:2[dBm] up to a maximum input power 9 Pin_(max): 7.5 [dBm] the behavior ofthe characteristic amplifier curve 7.OL is no longer linear. The entirecurve corresponds to a non-linear characteristic amplifier curve forwhich no predistorted signal is injected.

The lowest curve shows a characteristic amplifier curve 7.VL which hasbeen completely linearized. The shape of this characteristic amplifiercurve 7.VL is linear in the area of the input power of Pin: circa −12[dBm] up to a maximum input power 9 Pinmax: 7.5 [dBm]. Thislinearization of a non-linear characteristic amplifier curve 7.OL isachieved by the injection of a predistorted signal. In this casehowever, no increase of the output power can be achieved by signalpredistortion. It can be clearly seen from the diagram that thecharacteristic amplifier curve 7.VL over the entire area of approx. −15[dBm] to 7.5 [dBm] runs below the characteristic amplifier curve 7.OL.The output power Pout of the characteristic amplifier curve 7.VL is thuslower than the output power of the characteristic amplifier curve 7OL.However this complete linearization of the characteristic amplifiercurve 7.VL optimizes the signal quality.

The center curve shows a characteristic amplifier curve which has beenlinearized in accordance with the presently disclosed method and is onlypartly linearized 7.SML. This partial linearization, also known asSignal Matched Linearization, enables the output power Pout and theefficiency of a power amplifier to be increased over the entire range ofthe input power Pin. To only partly linearize the characteristicamplifier curve a look-up table is only used as from a specific inputpower Pin, namely the transition power 8 PinTH (6 [dBm] in FIG. 2). Thismeans that as from this transition power 8 PinTH entries are present inthe look-up table which multiply the signal amplitudes of the signalwhich are injected into the power amplifier by a factor of 1. Thelinearization of the characteristic amplifier curve is suppressed bythese entries in the look-up table. This can be clearly seen from theshape of the characteristic amplifier curve 7.SML in the area Pin_(TH):6 [dBm] to Pin_(max): 7.5 [dBm]. In this area the characteristicamplifier curve 7.SML is no longer linear. Although this partialsuppression of the signal linearization minimally reduces the signalquality and thereby the linearity of the characteristic amplifier curve,the output power Pout is however significantly increased compared to acompletely linearized characteristic amplifier curve 7.VL. This powerincrease can be seen from the shape of the characteristic amplifiercurve 7.SML. The characteristic amplifier curve 7.SML thus extends overthe entire input power range of Pin: approx. −13 [dBm] to Pin_(max): 7.5[dBm] above the characteristics of amplifier curve 7.VL. The gap betweenthe two characteristic amplifier curves in the diagram corresponds tothe power increase in [dBm].

FIG. 3—graphically depicts the values of a look-up table under theexemplary embodiment. In this graph the contributions of the complex I/Qsignal of the baseband 4.B are plotted on the abscissa. The associatedpredistortion factor is plotted on the ordinate. Between the points 0 to51 of the look-up table the predistortion factors are less than 1. Fromthe points 52 to 64 the predistortion factor is set to 1. Thus, frompoint 52 onwards, the signal is no longer predistorted. This“multiplication by 1” occurs in the curve 7.SML from FIG. 2 as from aninput power Pin of 6 [dBm].

FIG. 4 illustrates two measured curves under the exemplary embodiment,depicting the output power plotted against the frequency, with one curveshowing no predistortion and one curve showing complete predistortion ofthe signal respectively. The upper curve of the two curves shows ameasurement taken without predistortion. The lower curve shows ameasurement taken with predistortion.

In the lower part of FIG. 4, measured values for both curves are givenin a table. The measured values for the output power Pout, ACLR low andACLR up and the efficiency n are shown for the case of completepredistortion and that of no predistortion in lines two and three of thetable. The output power Pout and ACLR can be taken from the spectrum.The value for the efficiency n has been determined from the ratio of theoutput power Pout to the product of power consumption and voltageneeded. It can be seen from the table that the output power Pout and theefficiency n remain almost the same with and without predistortion. Onlythe ACLR has been able to be improved by the predistortion.

FIG. 5 illustrates two measured curves which depict the output powerPout plotted against the frequency with no predistortion and withpartial predistortion of the signals under the exemplary embodiment. Theupper of the two curves shows a measurement taken without predistortion.The lower curve shows a measurement taken with partial predistortion.

Like FIG. 4, measured values for the two curves are shown in the tableat the bottom of FIG. 5. The measured values for the output power Pout,ACLR low and ACLR up and the efficiency n are shown for the case ofcomplete predistortion and that of no predistortion in lines two andthree of the table. If the measured values from the Table in FIG. 4 arecompared to the measured values from the table in FIG. 5, it can be seenthat the output power Pout with partial predistortion amounts to 27.75[dBm] and has thus increased in comparison to the output power withoutpredistortion Pout, which amounts to 26.45 [dBm] (value from Table 1 ofFIG. 4). An increase in the output power Pout has thus been achieved bya partial predistortion. This also means that the efficiency n of theamplification has been increased by partial predistortion. From thetable of FIG. 4 a value of n=43.2 percent is produced for the efficiencywithout predistortion. From the table of FIG. 5, a value of n=50.2percent is produced for the efficiency with predistortion.

It should be noted that the value of the output power Pout withoutpredistortion can be manually increased so that the ACLR value can beobtained for the same output power.

Overall a method is thus presented by the invention which uses a partiallinearization of non-linear power amplifiers with predistortion and, inaddition to linear signal amplification, makes it possible to increasethe output power and also increase the efficiency of a power amplifier.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present disclosureand without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

1-4. (canceled)
 5. A method for improving the output power of anon-linear power amplifier comprising: predistorting signal amplitudesof a digital baseband; and injecting the signal amplitudes at an inputof the power amplifier, wherein the signal amplitudes are injectedpredistorted up to a predetermined transition power and, as from thispredetermined transition power, the signal amplitudes are injectednon-predistorted into the input of the power amplifier.
 6. The methodaccording to claim 5, wherein the signal amplitudes injected at theinput of the power amplifier are predistorted in such a way that thetransition power is set, with a linear part of the amplifier making thetransition to a non-linear part at this transition power.
 7. The methodaccording to claim 6, wherein the signal quality is adapted by settingthe transition power.
 8. The method according to claim 5, wherein alook-up table is used to predistort the signal amplitudes up to thepredetermined transition power and to inject the signal amplitudesnon-predistorted into the input of the amplifier as from thepredetermined transition power.